1. Field of the Invention
The present invention relates to a green phosphor and a device using the same, more particularly to a green phosphor capable of converting received light to light of lower energy (longer wavelength) and a device using the green phosphor. The phosphor of the present invention can be suitably used for gas discharge devices such as fluorescent lamps and displays such as plasma display panels (PDPs)
2. Description of Related Art
Phosphors are used in a variety of fields. For example, phosphors are used for luminaires such as fluorescent lamps, displays such as PDPs, and X-ray camera tubes.
Of such phosphors, Zn2SiO4:Mn is well known as a green phosphor which is excited by vacuum ultraviolet radiation. This phosphor is advantageous because it has high color purity (chromaticity coordinates: (0.21, 0.72)) and a high luminous efficiency. However, its luminance changes rapidly with time and its life is short. Also, when this phosphor is excited with strong light, the luminous efficiency drops and the luminance is saturated.
BaAl12O19:Mn, which is a known green phosphor, also has high color purity and high luminous efficiency, but has a short life.
Known phosphors having improved life and luminous efficiency are crystals having a magnetoplumbite-type structure with both a rare-earth element and a transition metal added as sensitizers to their luminescence center. Particularly, LaAl11O18:Eu2+, Mn (JJAP, 13(1974) pp. 950–956) and SrAl12O19:La, Eu2+, Mn (Philips Technical Review, 37(1977) pp. 221–233) are mentioned as old examples. With these phosphors, green light can be obtained by first obtaining blue light by exciting Eu2+ using suitable excitation light and exciting Mn2+ using the blue light. The blue light hardly comes outside because most of the blue light is used for exciting Mn2+.
In addition to the above-mentioned phosphors, SrAl12O19:Mn, Ln (Ln: a trivalent rare-earth element such as Ce3+, Pr3+, Gd3+, Tb3+) is known (U.S. Pat. No. 6,210,605). In this phosphor, energy transfers from the rare-earth element to Mn, and more green light can be obtained than a phosphor in which only Mn contributes to light emission.
Ce3+ is well known as a sensitizer element which intensifies light emission from Tb3+. For example, CeMgAl11O19:Tb is described in J. Luminescence, 9 (1974) pp. 415–419 and Philips Technical Review, 37(1977) pp. 221–233. In this phosphor, because the energy state of light emitted from Ce is almost equal to the energy state of f-d transition of Tb, energy transfers from Ce to Tb with high efficiency. This phosphor has a long life, but has a lower luminous efficiency than Zn2SiO4:Mn when excited by vacuum ultraviolet radiation. Further, the phosphor has low color purity (chromaticity coordinates: (0.33, 0.61)) because its emission spectrum has sub-peaks at 480 nm (blue, based on transition from 5D4 to 7F4), 580 nm (yellow, based on transition from 5D4 to 7F4) and 600 nm (red, based on transition from 5D4 to 7F3) in addition to a yellowish green emission line at 540 nm as a main peak (based on transition from 5D4 to 7F5). For this reason, this phosphor is not suitable for display devices.
A phosphor in which Tb is inserted in a borate (YBO3, LuBO3) containing a rare-earth element has a high luminous efficiency, but does not have good color purity, and therefore it is not suitable for display devices.
Japanese Unexamined Patent Publication No. HEI 5(1993)-86366 discloses a phosphor represented by (Ce1−xTbx) (Mg1−a−bZnaMnb)Al2zO2.5+3z (wherein 0<x≦0.6, 0<a+b≦1, 4.5≦z≦15). This phosphor has a spectrum of light emitted from Tb plus light emitted from Mn having a peak wavelength 515 nm. Therefore, the chromaticity is improved as compared with the above-described phosphors. However, regarding the light emission amount upon excitation by vacuum ultraviolet radiation, the phosphor is about 20% inferior to Zn2SiO4:Mn.